This invention relates to a high temperature heat resistant structure which is adapted to be used in a high temperature environment or in a flow passage of a high temperature gas turbine for providing structural walls, stationary or movable blades and the like.
A heat resistant structure heretofore used for providing structural walls or blades of a gas turbine has been constructed by use of a heat resistant metal plate I of a thickness t.sub.m, as shown in FIG. 1, one side surface I.sub.a of which is exposed to a high temperature fluid II of more than 1000.degree. C., while the other side surface I.sub.b of which is exposed to a coolant III such as cooling water.
The heat resistant structure of the above described construction, however, suffers from following difficulties a and b when it is used in a gas turbine for providing above described members.
a. An extremely high thermal stress is created in the metal plate I, thus reducing the operational life of the gas turbine.
b. Local boiling-up of cooling water tends to occur, thus reducing the cooling effect and the operable period of the structure.
The thermal stress .sigma. of the heat resistant metal plate I is proportional to the heat flux q flowing through the metal plate I and expressed as follows. EQU .sigma.=Ct.sub.m q (1)
wherein C is a constant determined by the material of the metal plate I. The heat flux q flowing through the metal plate I is on the other hand expressed as follows. EQU q=.alpha..sub.g (T.sub.g -T.sub.wout) (2)
wherein
T.sub.g represents temperature of the high temperature fluid, PA1 .alpha..sub.g represents heat transfer coefficient on the high temperature side of the metal plate I, and PA1 T.sub.wout represents surface temperature on the high temperature side of the metal plate I.
As is apparent from equation (2), the heat flux q increases in accordance with T.sub.g when the surface temperature T.sub.wout is maintained at its highest allowable value, the increase of q inevitably increasing thermal stress .sigma.. Although the thermal stress .sigma. can be restricted by reducing the thickness t.sub.m of the metal plate I as shown in equation (1), it is apparent that substantial reduction of the thickness t.sub.m is not practicable when the heat resistant structure is used under a high temperature and high pressure conditions.
In consideration of the local boiling-up of the cooling water, it is assumed that T.sub.win represents a surface temperature on the low-temperature side of the heat resistant metal plate I, and T.sub.sat represents a saturation temperature of the coolant III (cooling water in this case). A degree of superheat .DELTA.T.sub.sat is thus defined as follows. EQU .DELTA.T.sub.sat =T.sub.win -T.sub.sat ( 3)
It is apparent that the coolant III tends to be boiled-up when the degree of superheat .DELTA.T.sub.sat increases, and when the coolant boils-up, the advantage of providing a high heat conductivity .alpha..sub.C on the low-temperature side of the metal plate I is lost, and the cooling effect of the coolant III is substantially reduced.
To obviate the above described difficulty, the coolant III may be pressurized to increase the saturation temperature T.sub.sat and to reduce the degree of superheat .DELTA.T.sub.sat. However, since the coolant III must be pressurized at approximately 100 Kg/cm.sup.2 for achieving the above described object, a material of a high strength must be utilized for the construction of the coolant passage. As a consequence, the thickness of the heat resistant metal plate I must be increased, thus restricting the increase of the saturation temperature.
It is apparent that the boiling-up of the coolant may otherwise be prevented by reducing the surface temperature T.sub.win on the low-temperature side of the metal plate I. However, the surface temperature T.sub.win is expressed as ##EQU1## wherein .lambda..sub.m represents the heat conductivity of the metal plate I. Thus the reduction of the surface temperature T.sub.win inevitably increases the heat flux passing through the metal plate I so far as the temperature T.sub.g of the high-temperature fluid, the heat transfer coefficient .alpha..sub.g, and the thickness t.sub.m of the metal plate I are considered to be constant.
As is apparent from equation (2), although the heat flux q may be increased by reducing the surface temperature T.sub.wout on the high-temperature side of the metal plate I, the increase of the heat flux q inevitably increases the thermal stress .sigma. as defined in equation (1), and reduces the operational life of the metal plate I.
Although there has been proposed an arrangement wherein ceramic plates bonded together are provided on the high-temperature side surface of the metal plate I, such an arrangement tends to produce irregularities on the bonded surface of the ceramic plates on the high temperature side of the metal plate I, thus impairing smooth flow of the fluid on the side of the metal plate.